1. Field of the Invention
The present application relates to a recording material having a substrate and a negative-working, radiation-sensitive layer which includes a diazonium salt, metal-free organic colored pigments and a polymeric binder.
2. Description of the Related Art
Recording materials for offset printing plates are known. European Patent Publication No. 152 819-A (corresponding to U.S. Pat. No. 4,631,245) describes a negative-working recording material for the production of offset printing plates where the photosensitive layer contains a diazonium salt polycondensate, a carboxyl-containing polymeric binder and, optionally, a dye and/or colored pigments. For example, a paste, obtainable from Clariant Deutschland GmbH, under the name .RTM.Renolblau B2G-HW, which is a blue copper phthalocyanine pigment (.RTM.Hostapermblau B2G) in a modified polyvinyl butyral (.RTM.Mowital B30H) is used for coloring the layer. However, the pigments pretreated in this manner dissolve slowly and to a limited extent in the coating solution. This limits the pigment content and hence the subsequently achievable image contrast. By increasing the stirring time in the mixing vessel, it is possible to achieve better mixing, but by increasing the stirring time, undesired sludge formation occurs. Copper-containing pigments also leave an undesired accumulation of copper in the aqueous baths where the recording materials are developed after imagewise exposure. Moreover, copper phthalocyanine pigments, as well as polyvinyl butyrals, only dissolve to a limited extent in aqueous alkaline developers, so that precipitates of layer components frequently form when the developer is highly contaminated. The precipitates in turn may impair the function of the developing machines as well as that of the developed plates by the formation of redeposits.
European Patent Publication No. 778 497-A relates to a negative-working recording material suitable for the production of offset printing plates. The unexposed parts of the radiation-sensitive layer of the recording material can be removed using neutral or alkaline aqueous solutions. The radiation-sensitive layer contains predispersed pigments, such as predispersed copper phthalocyanine pigments, a diazonium compound or a combination of a photopolymerizable compound and a photoinitiator. The radiation-sensitive layer also contains a polymeric binder which is soluble, or at least swellable, in aqueous alkaline solutions. The binder is prepared by reacting a hydroxyl-containing polymer with an anhydride, such as maleic anhydride or succinic anhydride, in the presence of a catalytically active amount of a tertiary amine. Anhydride-modified polyvinyl acetals are disclosed as suitable binders. The acid number of the binder is in general from 5 to 80. The binder also serves to disperse the colored pigments. In the pigment dispersion, the amount of the binder is from 2 to 20% by weight. In addition, the pigment dispersion includes from 60 to 96% by weight of an aliphatic (C.sub.1 -C.sub.8)-alcohol, a (C.sub.1 -C.sub.8)-alkylene glycol mono- or di-(C.sub.1 -C.sub.8)-alkyl ether (in particular 1-methoxypropan-2-ol), or a cyclic ketone. As a result of the predispersing, the amount of the colored pigments in the layer can be increased, thereby increasing the image contrast in the developed recording material and making the image more visible. This also ensures that the layer is completely removed in the unexposed parts and that the detached layer components are not redeposited, which would lead to a background fog or scumming during subsequent printing. The predispersing of the pigment is usually carried out in a ball mill. If an organic solvent having a relatively low boiling point is used (such as 1-methoxypropan-2-ol, which has a boiling point of from 118 to 119.degree. C. at atmospheric pressure), however, thorough cooling of the mill is absolutely essential.
Attempts have also been made to eliminate the deficiencies of the recording materials disclosed in the prior art. European Patent Publication No.738 931-A discloses coloring a negative-working layer with heavy metal-free, in particular copper-free, predispersed phthalocyanine pigments. The predispersal is achieved with a polyvinyl alcohol in water, in the presence of a cationic surfactant. Although stable dispersions can be produced in this way, they can only be used in aqueous media. The pigment dispersion according to this European Patent Publication is, accordingly, only used in an aqueous photosensitive composition which, in addition to a water-soluble diazo resin or diazonium salt, contains a polyvinyl alcohol as a binder. The high polyvinyl alcohol content in the layers frequently leads to problems with ink acceptance during printing. Furthermore, the layers are not sufficiently resistant to the fountain solutions used during printing, and thus only relatively short print runs can be achieved. Also, in photosensitive mixtures which contain water-insoluble diazo or diazonium components and/or water-insoluble polymeric binders, the pigment dispersion is virtually unusable.